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There are tons of variables that play into it, and they all work together (or against each other if not properly matched). Porting is a big part. The port length and diameter changes where the peak power will be. Shorter larger diameter intake ports tend to make little power at low RPM's and more power at high RPM's. Longer, smaller diameter ports tend to make more power at low RPM's, but then fall flat at high RPM's. Obviously there are limits to how small or how large you cam make the ports. Cam timing is another big player. Where the lobe centers are located can define where and how the power is produced. Advancing the cam timing tends to shift the peak power more towards low RPM, retarding the cam timing tends to shift it more towards high RPM. Also changing the separation angle of the intake and exhaust cams makes a difference. Making the angle smaller tends to make a peakier powerband, widening it tends to decrease your peak power, but spreads the power out over a larger RPM range. A tuned exhaust also plays a crucial role. As the exhaust leaves the pipe, it's momentum produces a vacuum effect inside the combustion chamber. During the time when the exhaust valve is still closing, and the intake valve is opening this vacuum effect (called exhaust scavenging) helps pull the intake charge in. Different length and diameter exhausts change the RPM range that the scavenging effect is the strongest in, and thus increase power at these RPM's. There are a lot of other variables that come into play, like ignition timing, rod length, compression ratios, etc that also affect an engine's operation, but these are the basics.

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Only thing I could add is that in conjunction with intake tract/port diameter, is that stroke to bore ratio changes the PHYSICAL reactions to design.

An oversquare motor (shorter stroke/ larger bore) will spin more readily being that the distance it has to travel to make a revolution is shorter which means the WORK was technically less. This leads to a more powerful top end being that it can spin so readily to high rpm.

An undersquare motor (long stroke/small bore) will spin slower because the distance the piston moves to make a revolution is greater which requires more work. The positive effect of this is momentum and interial forces which produce more/slower torque forces in the forward acceleration and deceleration of the engines crankshaft.

A square motor (almost equal bore/stroke) give a better blend of the low end power and top end rush that can make a motor feel "easier" or "more usable" than it's short or long stroke counterparts.

The intake and exhaust must match your motor at optimal rpms to make HP. FLOW IS KING.

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Only thing I could add is that in conjunction with intake tract/port diameter, is that stroke to bore ratio changes the PHYSICAL reactions to design.

An oversquare motor (shorter stroke/ larger bore) will spin more readily being that the distance it has to travel to make a revolution is shorter which means the WORK was technically less. This leads to a more powerful top end being that it can spin so readily to high rpm.

An undersquare motor (long stroke/small bore) will spin slower because the distance the piston moves to make a revolution is greater which requires more work. The positive effect of this is momentum and interial forces which produce more/slower torque forces in the forward acceleration and deceleration of the engines crankshaft.

A square motor (almost equal bore/stroke) give a better blend of the low end power and top end rush that can make a motor feel "easier" or "more usable" than it's short or long stroke counterparts.

The intake and exhaust must match your motor at optimal rpms to make HP. FLOW IS KING.

I hope this made sense. -BIG DAN:thumbsup:

This makes sense. I can feel the air coming out of my YZ250F is intense compared to my wife's TTR230. I wonder how much of that YZ250F's increased volume is due to more flow/compression vs the combustion. I guess flow would be a factor of compression and combustion.

Also, is a 2 stroke of 4 stroke more of a square motor? Are 250F's with under-square motors better for being bored out?

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There are tons of variables that play into it, and they all work together (or against each other if not properly matched). Porting is a big part. The port length and diameter changes where the peak power will be. Shorter larger diameter intake ports tend to make little power at low RPM's and more power at high RPM's. Longer, smaller diameter ports tend to make more power at low RPM's, but then fall flat at high RPM's.

OK, this makes perfect sense. I remember on my jetski, a wide nozzle was better for acceleration and a narrow nozzle was better for top speed. This is the same in carbs and rocket nozzles isn't it?

Do 4 stroke exhausts use resonance like a 2 stroke? I remember you could tune a jetski exhaust with water...

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oooboy.. whether a motor is a 2 stroke or a 4 stroke has nothing to do with the bore/stroke ratio. In general, all dirt bikes are under squared because more torque is ideal for a dirt surfaces and it allows them to work well in low speed trials.

Boring out a cylinder ( and porting respectivly) will give you similiar a power curve, just a steady increase of hp throught the revs. Wether a cylinder is undersquared or over squared initialy, a big bore kit will have the same effect.

Keep in mind, weather a motor is undersquared or over squared, it still will the same mean piston speed.

a carb with a small bore diameter is good low end. think of straw, the narrower it is the easier it will be suck up what ever your drinking. a large bore is good for high end power. Once the revs starts building up the amount of air needed increases. A small bore carb cannot provide the same amount of air as a large bore carb.

As far as i know 4 stroke pipes do use resonance, because it increase scavaging.

There is tons of facts on this online, thats where i learned

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Yes, intake tracts and exhaust systems both use resonance to increase power on both a two stroke and a four stroke engines. The idea is to optimize when the pressure wave is hitting near the combustion chamber to the RPM range you want to operate in. If done right you can get intake pressures above atmospheric pressure when the intake valve opens, which forces air in, as well as negative pressure in the exhaust during the overlap period that will help suck the exhaust out and the new intake charge in. Changing the length and volume of the intake or exhaust changes the resonance frequency, and thus changes the RPM range where this effect is optimized.

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What about ignition? When I was getting my private pilot, all the airplane engines used dual spark plugs. There was a noticeable RPM change when you ran one plug instead of two. Why don't they run dual plugs on dirtbikes? Not enough room with all the valves?

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What about ignition? When I was getting my private pilot, all the airplane engines used dual spark plugs. There was a noticeable RPM change when you ran one plug instead of two. Why don't they run dual plugs on dirtbikes? Not enough room with all the valves?

Now this IS MY area of expertise.

Being an A+P mechanic I must know.

Aircraft can't pull over when the ignition coil fails. Thusly redundancy is built into the engine using 2 completely different systems. When you run on just one, all you are doing is testing each system independently.

Now as far as the RPM change is concerned,

FLAME FRONT AND PISTON SLAP.

The flame can only move so fast across the piston face if the spark plug is on one side or the other of the cylinder. Using two spark plugs makes TWO flame fronts which meet at piston center causing a much more efficient (when timed correctly) PUSH on the piston. If its too early you get piston slap which is not good for making power.

4 cylinders from the 80s and 90s have two ignition systems run from the same coil input wire. For efficiency of burn and emission control.

On aircraft its for saftey.

-BIG DAN:thumbsup:

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This makes sense. I can feel the air coming out of my YZ250F is intense compared to my wife's TTR230. I wonder how much of that YZ250F's increased volume is due to more flow/compression vs the combustion. I guess flow would be a factor of compression and combustion.

Also, is a 2 stroke of 4 stroke more of a square motor? Are 250F's with under-square motors better for being bored out?

generally four strokes are always an over square engine, 250F's generally run around a 75-79mm bore with a 50-53mm stroke, while 450's run around a 60-63mm stroke and 94-97mm bore, two strokes on the other hand are completly differant, almost all of the last generation of 125's ran a 54mm stroke with a 54.5mm bore (square bore and stroke), while all of the 250 stroke ran a 72mm stroke with a 64mm bore (in otherwords an under square engine), and all 300 two stroke are square with 72mm bore and stroke (in otherwords a big bore 250 two stroke), cr 500's are a 79mm stroke to a 90mm bore while a kx 500 are 84mm bore and stroke.

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Yes, intake tracts and exhaust systems both use resonance to increase power on both a two stroke and a four stroke engines. The idea is to optimize when the pressure wave is hitting near the combustion chamber to the RPM range you want to operate in. If done right you can get intake pressures above atmospheric pressure when the intake valve opens, which forces air in, as well as negative pressure in the exhaust during the overlap period that will help suck the exhaust out and the new intake charge in. Changing the length and volume of the intake or exhaust changes the resonance frequency, and thus changes the RPM range where this effect is optimized.

Expansion chambers on 2t do not operate the same as pipes on 4t.

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Of course not, as the engines are very different designs. However, the exhaust system on both a four stroke and two stroke is designed to use resonance of pressure waves to the engine's advantage, just in different ways to meet the needs of the particular engine design.

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Their were old dirt bikes with 2 spark plugs, more for reason of fouling with weak igntions and dirty oils of the day. VS. the wide bores of aircraft engines and the time it take the flame front to travel across it.

The hot setup with the old dual plugs bikes was to remove 1 and install a manual compression release kit. Like a jake brake on a truck you'd use it slow the bike coming off a hill etc. Since the drum brakes were so bad back then when wet, it was a great addition.